Disc refiners are utilized in papermaking to prepare wood fibers to be made into paper on a papermaking machine. Disc refiners are generally divided into two types: those for refining high consistency stocks containing 18 to 60 percent fiber by weight; and those for refining low consistency stocks having two to five percent fiber by weight. High consistency refiners produce mechanical and semi-mechanical pulp or furnish from undigested wood chips and semi-digested wood chips. These refiners break down the wood chips and clumps of wood fibers into individual fibers from which paper is formed.
Processing of fibers in a low consistency refiner may be performed on both chemically and mechanically refined pulps, and in particular may be used sequentially with a high consistency refiner to further process the fibers after they have been separated in the high consistency disk refiner. In operation, a low consistency disc refiner is generally considered to exert a type of abrasive action upon individual fibers in the pulp mass so that the outermost layers of the individual cigar-shaped fibers are frayed. This fraying of the fibers, which is considered to increase the freeness of the fibers, facilitates the bonding of the fibers when they are made into paper.
Paper fibers are relatively slender, tube-like structural components made up of a number of concentric layers. Each of these layers (called "lamellae") consists of finer structural components (called "fibrils") which are helically wound and bound to one another to form the cylindrical lamellae. The lamellae are in turn bound to each other, thus forming a composite which has distinct bending and torsional rigidity characteristics. A relatively hard outer sheath (called the "primary wall") encases the lamellae. The primary wall is often partially removed during the pulping process. The raw fibers are relatively stiff and have relatively low surface area when the primary wall is intact, and thus exhibit poor bond formation and limited strength in the paper formed with raw fibers.
It is generally accepted that it is the purpose of a pulp stock refiner to partially remove the primary wall and break the bonds between the fibrils of the outer layers to yield a frayed surface, thereby increasing the surface area of the fiber multi-fold.
Disc refiners typically consist of a pattern of raised bars interspaced with grooves. Paper fibers contained in a water stock are caused to flow between opposed refiner discs which are rotating with respect to each other. As the stock flows radially outwardly across the refiner plates, the fibers are forced to flow over the bars. The fiber treating action is thought to take place there, between the closely spaced bars on opposed discs. It is known that sharp bar edges promote fiber stapling and fibrillation due to fiber-to-fiber action. To achieve this, an advantageous method of fabricating bars which wear sharp has been utilized in the construction of refiner plates such as disclosed in U.S. Pat. No. 5,165,592 to Wasikowski. It is also known that dull bar edges result in fiber cutting by fiber-to-bar action. Fiber cutting is undesirable because it results in paper of weaker strength and renders a certain portion of the fibers too small to be retained on the screen on which the paper is formed, thus increasing waste.
The preferred action in refining paper fibers is fibrillation. Fibrillation is the breaking down of the primary wall and partially releasing the fibrils of the outer layer to yield the frayed surface, which increases the surface area of the fiber multi-fold. Improved fibrillation with minimal fiber damage has been theorized as possible if a refiner bar having a rough or abrasion resistant edge is used. The rough or abrasion resistant edge, which resists dulling during operation, holds the fibers longer while the sharpness of the rough surface acts to gently abrade the fibers. A rough or abrasion resistant edge is difficult to obtain without affecting all of the surrounding surfaces. If all of the surrounding surfaces are treated, fiber flow through the refiner may be impaired by the loss of open area in the grooves between the refiner bars as well as by the added friction of the abrasive material. Treatment of the entire groove and treatment of the bar surface have been accomplished by surface modification techniques but the edge has not been isolated.
Both theory and logic suggests that work is being done to wood fibers passing through a refiner principally as the fibers pass over the outermost surface of the bars. Thus, it is desirable to retain the fibers on the outermost surface and to build up a fiber pad thereon to promote refining. One way to retain fibers on the outermost surface of refiner bars is to make the surface rough. The roughness creates numerous edges to hold the fibers so that they may be refined.
There are many ways of depositing a rough surface or other coating on a refining plate bar, but these have all involved adding thin layers of material on top of the bars after they have been finished because the bar surfaces must be ground to obtain flatness and bar depth requirements. Thus, the problem associated with depositing a rough surface or other coating on the outermost surface of the refiner bars is that, on the one hand, it can affect the flatness of the bars, which interferes with the ability to run opposed discs closely spaced; and on the other hand, there is a tendency of the relatively thinly deposited layer to rapidly wear away during operation in a refiner.
What are needed are techniques for creating localized areas of surface roughness which resist wearing away.